Montana Field Guide

The Black Sandshell is the largest mussel found in the Missouri and Milk river systems. It has a very heavy, large long-elliptical black shell (up to 10 inches) with a pinkish to purple nacre. It is likely an introduced species to Montana because it was not found or reported before the 1970's and now appears to be relatively abundant and increasing it's range up the Missouri River, but not in the Yellowstone River.

"Shell up to about 175 mm long, 75 mm high, 45 mm wide (males) and with mid-anterior shell wall about 8 mm thick. Females are proportionately broader, especially posteriorly. Shell long-elliptical, roundly pointed posteriorly in males, evenly rounded posteriorly in females, thick and strong. Surface smooth except for concentric wrinkles and growth rests. Periostracum dark green, dark brown or nearly black, and with darker rays that are obscure in adults. Nacre silvery white throughout, or partially to wholly purple or pink. Beak sculpture faint, obsolete, and consisting of 3 to 5 indistinct double-looped bars that disappear on the posterior slope. Hinge teeth well developed and strong: pseudocardinal teeth conical, elevated, serrated, 1 or 2 in the right valve and 2 in the left; lateral teeth long, also elevated 1 in the right valve and 2 in the left. Easily recognized by its large size, lance-head shape, well-developed hinge teeth, and almost black colour." (Clarke 1981:334).

Adult freshwater mussels are mostly sedentary and spend their lives near the area where they first settled as juveniles. They will move short distances (a few meters) during spawning or if prompted by a stimulus such as a water disturbance nearby, exposure due to low water, or seasonal temperature changes. They may burrow into sediments (often doing so in the fall and emerging in the spring) or move away from the area horizontally. Most dispersal takes places through there host fish, especially the spring/summer movements of their host fish walleye and sauger.

The species usually occurs in large rivers but is also found in large lakes and canals. It is generally found on sand and gravel bottoms, but occasionally on mud (Clarke 1981:334). In Montana, this species was found in areas with higher percentage of cobble/pebble than co-occuring fatmuckets in the Missouri, Milk and Marias River systems and never in silted areas (Stagliano 2009)

Freshwater mussels are mostly filter-feeders, siphoning in floating particulate organic materials (small plant or animal) from the water column and straining out the particles and expel the strained water. Pedal feeding with the foot muscle has also been observed, mostly in juveniles and younger age-classes.

Known host fish species include the native Sauger and introduced species: the Walleye and some centrarchids (sunfish).

"A long-term breeder; its gravid period overlaps in different individuals and extends from June, July and August through the winter to the following summer. The glochidia are purse-shaped, ovate, and measure about 0.23 mm long and 0.27 mm high. The known fish hosts include the native sauger and introduced centrarchids (bluegill, largemouth bass, and white crappie) (Clarke 1981:334).

Threats to freshwater mussels and their habitats are thought to include: Pollution, Diversion of rivers for irrigation, hydroelectric and water supply projects, Elimination of natural fish hosts and eutropification due to agricultural runoff and urbanization